Lead monoxide plate for X-ray electro-photography and method of preparing same



Aug. 16, 1966 SURFAC E VO LTAGE (VOLTS) TOO-- GOO-- ZOO-- E. s. ANOLICK 3,266,932

LEAD MONOXIDE PLATE FOR X-RAY ELECTROPHQTOGRAPHY AND METHOD OF PREPARING SAME Filed Aug. 6, 1965 SELENIUM PLATE SAMPLE 5 I50 VOLTS DIFFERENCE @5 SECONDS 3.5 SECONDS DIFFERENCE @450 VOLTS l l I I I l0 I5 20 25 3O TIME OF lRRADtATlON (SECONDS) United States Patent 3,266,932 LEAD MONOXIDE PLATE FOR X-RAY ELECTRO- PHOTOGRAPHY AND METHOD OF PREPARING SAME Eugene S. Anolick, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Filed Aug. 6, 1963, Ser. No. 300,245 7 Claims. (Cl. 117-201) The present invention relates to an X-ray electrophotographic plate which utilizes lead monoxide as the X-ray sensitive photoconductive element and further relates to a method of preparing such plates.

The X-ray electrophotographic process comprises, in general, the steps of charging the surface of a plate which contains an X-ray sensitive photoconductive element; irradiating the charged surface with the pattern of X-rays which is to be reproduced so that the X-rays discharge the irradiated areas by photoconduction; placing the exposed plate in a powder cloud to collect powder on the charged areas and finally, fixing or photographing the resulting plate. Previously known plates used in this process have generally used selenium as the photoconductive element because of the relatively high photoconductivity of selenium under X-radiation. However, it has been found that these plates have not been of sufficient speed to distinguish details of the X-ray patterns of human and other subjects which must be studied.

The present invention is therefore directed to the provision of a new and improved plate for use in the X-ray electrophotographic process which overcomes the difliculty of low speed of previously known plates and to a method of preparing such plates.

It is accordingly an object of the present invention to provide a new and improved high speed plate for use in the X-ray electrophotographic process.

A further object of the present invention is the provision of a new and improved X-ray electrophotographic plate which is highly sensitive to X-radiation in irradiated areas without being affected in non-irradiated areas.

Another object of the present invention is the provision of a new and improved method of preparing plates for use in the X-ray electrophotographic process.

Further objects and advantages of the present invention will become apparent as the description and illustration thereof proceed.

Briefly, in accordance with one form of the present invention, a quantity of lead monoxide of orthorhombic crystal structure is placed in a dry air or dry inert gas atmosphere and fired. The lead monoxide is then placed in an inert gas atmosphere and mixed with an appropriate insulating binder. The resultant suspension, still in the inert atmosphere, is coated on a conductive surface or backing member and dried, forming an X-ray electrophotographic plate. The preparation of the plate may also include the steps of adding appropriate compounds to increase or decrease the conductivity of the lead monoxide or to prevent the formation of pockets of charge in the irradiated areas. Plasticizers and/ or wetting agents may also be added to the suspension for handling or coating purposes.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the appended drawing which is a comparison of results obtained with a plate prepared in accordance with the present invention and with a previously known plate.

In the preparation of the X-ray electrophotographic plate of the present invention, the lead monoxide (PbO) "ice utilized is of the orthorhombic form. The lead monoxide is provided in powdered form and it has been experimentally determined that the preferred range of particle sizes is 0.25 to 10 microns and, within this range, opti mum results have been achieved with particle sizes between 0.5 and 5 microns. The lead monoxide is then heat treated or fired in a dry atmosphere to dry out surface water vapor in theparticles. The heat treatment may be in the range of 300 C. to 600 C., preferably in an inert atmosphere. Alternatively, the lead monoxide may :be fired in dry air. In this case, excess oxygen is introduced into the powder to provide P-type conduction; that is, conduct-ion due to the presence of empty energy states which permit electron movement. It is noted that, if the heat treatment is carried out in air, the range should be either C. to 350 0., preferably between 250 C. and 350 C. or in the range of 500 C. to 700 C., preferably between 550 C. and 650 C. The intermediate range, between 350 C. and 500 C. may not be used with an air atmosphere because of the transformation from lead monoxide to higher oxides of lead which are detrimental to the operation. It is noted that, if the range above 500 C. is used, the lead monoxide must be rapidly cooled through the prohibited range to inhibit this transformation.

Following such heat treatment, other oxygen-content controlling compounds may be introduced if it is desired to control the amount of excess oxygen and thereby increase or decrease the amount of P-type conduction. For example, oxidizing or reducing compounds could be used to respectively increase or decrease the amount of oxygen. If such conduction is not desired in the finished plate, an inert atmosphere may be provided through-out the heat treatment so that the stoichiometric ratio of lead to oxygen is maintained.

After the powder has been heat treated, it is mixed with a solution of an appropriate insulating :binder. The binder must be of a material which will not react with the lead monoxide and further must be sufficiently insulat-ing so that it will not conduct away the charge pattern left in the lead monoxide after irradiation of the finished plate by X-rays. An appropriate material is a resin, such as an isobutyl methacrylate polymer, for example, that manufactured by E. I. Dupont de Nemours and Company, Incorporated, and sold under the tradename Lucite 45. Before mixing the binder with the powder, the resin is placed in solution, for example a 10% solution of the resin in toluene. The powder is added to the solution and the completeness of the distribution of the powder in the solution is insured by ball milling for an extended period of time, for example, two hours, in an ultra high speed mill with stone or porcelain balls. The resultant mixture is a suspension of lead monoxide in the solution. The weight ratio of lead monoxide to resin in the solution is in excess of 4:1 and preferably between 4:1 and 16: 1. Plasticizers or wetting agents may also be added to the solution before ball milling to facilitate handling and application of the coating, but the usefulness of such additives is limited due to the possibility of increased conductivity resulting from their presence.

The prepared solution is coated by hand with a doctor blade on a conductive backing member which may comprise, for example, aluminum foil or conducting paper. The solution is then dried to complete the plate of the present invention. It has been found that the coating thickness should be between /2 and 10 mils, with optimum results being obtained from thicknesses between 1 and 5 mils. Thicker coatings require too long a time for the exposed areas of the plate to discharge while uniform thinner films are difficult to achieve because the thickness approaches the particle size.

Curling of the aluminum foil may make it difficult to achieve a uniform coating. This may be overcome by spreading silicone oil on a flat glass plate and then flattening the foil in the oil. The coating may then be performed uniformly with less difficulty.

In the case of coatings having high concentrations of lead monoxide, it was found that uniform wetting of the aluminum surface may be facilitated by first coating the foil with a vinyl acetyl material in solution in chloroform by weight) in a very thin layer. The lead monoxide coating is then applied.

In developing the irradiated plates, the well-known powder cloud technique is preferred where pictures with high resolution are required. In situations with less stringent requirements, the magnetic brush technique, also well-known, may be used. This technique is less desirable where high resolution is needed since a constant background level exists which may be due to a triboelectric effect between iron and lead monoxide.

The novel article of the present invention is thus an X-ray electrophotographic plate comprising a conductive backing member and a coating of lead monoxide in an appropriate binder. In the utilization of this plate, the surface is charged and then exposed to a pattern of X- rays determined by the object being photographed The X-rays cause p'hotoconduction to occur in the exposed plate area, thereby discharging them while leaving the charge on areas in the shadow of the object. The exposed plate is then placed in the powder cloud so that the powder adheres to the charged areas and the resultant representation of the object is developed.

The remainder of this specification is directed to illustrations of the advantages gained through the use of lead monoxide plates prepared in accordance with the present invention. The following samples, prepared in the manner indicated, were used to measure the data recorded below:

Samples 8 -8 were prepared in weight ratios of lead monoxide-to resin of, respectively, 1:1, 2:1, 4:1 and 8:1 using orthorhombic lead monoxide obtained from the Evans Lead Corporation and heat treated at 300 C. for one hour in air. The particle size range of the lead monoxide was 0.5-5 microns, the main portion being between 0.5-2 microns. The purity of the lead monoxide is important since impurities in the powder may cause undesired conduction resulting in poor quality pictures. A spectographic analysis revealed the following maximum limits of quantities of impurities in the lead monoxide obtained from the Evans Lead Corporation:

Percent Bi 0.03 Sn 0.0001 Fe 0.0005 Zn 0.0005 Au 0.0002 Te 0.001 Cr 0.0001 Cu 0.0006 As 0.0006 Mi 0.0001 Mn 0.00005 Pt 0.0001 Tl 0.0001 Al 0.0005 Ag 0.001 Sb 0.001 Cd 0.0008 Co 0.0001 Ti 0.0001 Mg 0.0002

The remainder of the preparatory steps were as de scribed above.

Sample S was prepared in the ratio of 16:1 from a quantity of orthorhombic lead monoxide, reagent grade,

obtained from the Fisher Company, and heat treated in air at 305 C. for two hours, the remainder of the preparatory steps being as described above. The estimated particle size range was 215 microns, the main portion being in the range of 2-5 microns. The analysis supplied with the Fisher Company lead monoxide listed the following maximum quantities of impurities:

lPercent N0 0.007 Cl 0.002 Fe 0.004 Bi 0.01 Cu 0.002 Ag 0.0002 Alkalies and earths 0.05

Sample S was also prepared from the reagent grade orthorhombic lead monoxide obtained from the Fisher Company. The powdered lead monoxide was mixed with 2% lead carbonate and the mixture was fired in air at 350 C. for one hour, the remainder of the preparatory steps being as described above.

The following table indicates the speeds of several samples of lead monoxide plates in accordance with the present invention. The speeds were measured relative to that of selenium which was considered to be one.

Table I Sample Relative speed 5 .35 S .5 s 1.2 8.; 1.7

It is noted that the samples 5 -5 in the above table are arranged in order of increasing weight ratio of lead monoxide to resin. It therefore appears from the above table that lead monoxide plates prepared in accordance with the above described method and prepared with a sufficiently high ratio of lead monoxide present in the plate will provide substantially higher speed than the standard selenium plate previously known. These figures represent averages over several regions of the sample. In one instance, a speed approximately ten times that of selenium was obtained.

Table II indicates the effect of film thickness on the relative speed:

Although the figures given in this table are not perfectly progressive, they indicate the fact of increasing speed with decreasing film thickness. The limit of high speeds is, of course, reached when the film thickness approaches the diameter of the individual lead monoxide particles. While it is not intended to limit the present invention to the following explanation, it appears on the basis of present considerations that a thick film requires a longer time for the lead monoxide to properly distribute itself. Also, there is the obvious factor of the length of time for the X-rays to impart their pattern to the complete thickness of the coating. Regardless of the precise theoretical explanation, it has been found that high weight ratios and thin films will generally provide improved results.

The advantage of the lead monoxide plates of the present invention over the selenium plates previously known is further illustrated by the attached drawing, which is a plot of the decay of surface voltage under irradiation by an X-ray beam. It can be seen from this graph that, while after a long period of time such as 2030 seconds,

the sample S and the selenium plate show approximately the same discharge, in time intervals less than 20 seconds the sample prepared in accordance with the present invention is significantly faster than the selenium plate previously used. For example, after five seconds of irradiation, the surface voltage of the selenium plate had only dropped to approximately 550 volts While the surface voltage of the lead monoxide plate had dropped to 400 volts. The selenium plate took 3.5 seconds longer to reach 450 volts, a useful value for the powder cloud technique, than did the lead monoxide plate. This decrease in the exposure time can be very significant in the case of relatively delicate samples. Thus, X-ray photographs of equal or better quality than the X-ray photographs taken using the previously known selenium plate can be made with a substantial reduction in the length of time of radiation by the X-ray beam. This is obviously advantageous in terms of reduced time necessary to operate the beam and also a reduction in the time during which any individual object is subjected to irradiation.

Theoretical considerations also indicate a further advantage of the lead monoxide plates of the present invention which may appear at higher energy X-ray beams. Although this was not specifically tested, the absorption cross section of lead monoxide increases sharply relative to that of selenium for photon energies in excess of 100 kilovolts. Thus, X-ray beams having peak energies in excess of 100 kilovolts should result in substantially higher speeds for lead monoxide plates as compared to those possible with the presently known selenium plates.

In general, it can be seen that the use of the lead monoxide plate of the present invention, prepared in accordance with the above described method, results in an X-ray electrophotographic plate having a significantly higher speed than that of previously known plates. Important factors are the purity of the lead monoxide, the use of orthorhombic lead monoxide, and the use of coatings having high weight ratios applied in relatively thin films. Also, the particle size of the lead monoxide should preferably be in the range described above.

The specific embodiments described herein are presented merely as examples of the various forms which this invention may take. appended claims to cover all modifications and variations which may come within the true spirit and scope of this invention.

I claim:

1. A process of preparing an X-ray electrophotographic plate comprising the steps of firing a quantity of orthorhombic lead monoxide in a dry atmosphere; mixing said lead monoxide with an insulating binder in said dry atmosphere; and coating a conductive member in said dry atmosphere with said mixture.

2. The process claimed in claim 1 wherein said lead monoxide is provided in powdered form, the diameters of the particles in the powder being less than 10 microns.

3. The process claimed in claim 1 wherein said atmosphere comprises an inert gas.

4. The process claimed in claim 1 wherein said atmosphere contains oxygen.

5. The process claimed in claim 4 including the step of adding oxygen-content controlling compounds to said lead monoxide after said firing is completed.

6. The process claimed in claim 1 wherein the proportion by weight of said lead monoxide to said binder is in excess of 4:1.

7. The process claimed in claim 1 wherein the thickness of the coat is between 0.5 and 10 mils.

References Cited by the Examiner UNITED STATES PATENTS 2,888,370 5/1959 Damon et al. 1l7201 2,997,387 8/1961 Tanenbaum 252-501 3,008,825 11/1961 Van Dorn 96-1 3,082,085 3/1963 Miller 250651 X RALPH G. NILSON, Primary Examiner.

H. S. MILLER, A. L. BIRCH, Assistant Examiners.

Therefore, it is intended in the 

1. A PROCESS FOR PREPARING AN X-RAY ELECTROPHOTOGRAPHIC PLATE COMPRISING THE STEPS OF FIRING A QUANTITY OF ORTHORHOMBIC LEAD MONOXIDE IN A DRY ATMOSPHERE; MIXING SAID LEAD MONOXIDE WITH AN INSULATING BINDER IN SAID DRY ATMOSPHERE; AND COATING A CONDUCTIVE MEMBER IN SAID DRY ATMOSPHERE WITH SAID MIXTURE. 